Plasma doping (PLAD), also referred to as plasma immersion ion implantation (PIII), is a well known technique for implanting dopants into a substrate.
FIG. 1 shows a typical PLAD system 100. The system 100 may comprise a process chamber 102 having a platen 104 to hold a wafer 12. One or more reactive gases may be fed into the process chamber 102 via a gas inlet 106 through a top plate 108. The reactive gas(es) may then be distributed uniformly through a gas baffle 110 before entering the process chamber 102. A group of coils 112 may couple radio frequency (RF) electrical power into the process chamber 102 through an aluminum oxide (Al2O3) window 114. The RF power may produce a dopant-containing plasma discharge 10 from the reactive gas(es). A bias voltage may be applied to the wafer 12 or the platen 104 to draw charged particles from the plasma discharge 10. Typically, the wafer 10 is biased with pulsed DC voltage while the plasma is always on. As a result, dopant ions are extracted into the wafer only during the pulse-on period.
Ideally, excessive charged particles delivered to a wafer should be promptly neutralized by electrons supplied from a plasma discharge. Unfortunately, the electrical contact between the wafer and the platen may be a poor one, and the wafer may have electrically isolated portions (e.g., oxide, nitride, and/or photoresist), both of which may contribute to charge buildup on the wafer. If a sufficient amount of charge is collected on the wafer, there may be enough potential difference between the wafer substrate and other cathode components to cause arcing in the plasma chamber, which is extremely damaging to microelectronic devices.
Existing methods for detecting wafer charging are mostly developed for beam-line type ion implanters and are not suitable for PLAD or other plasma-based systems. One existing approach employs wafer potential probes which are unreliable because they suffer from the same problem of poor electrical contact as wafer backside contacts. Another approach involves localized detection of photoresist outgassing, which only senses conditions conducive to arcing but does not directly measure wafer potential (or changes thereof) in a reliable way.
In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with current wafer charging detection technologies.